feat(agent-plugin): add official RxJS agent plugin

[benlesh]

Summary

• add the official @rxjs/agent-plugin beta with 13 version-aware skills, a portable Agent Plugins manifest, and a digest-locked Claude adapter
• move the deterministic RxJS 7-to-9 migration engine behind a read-only, source-text-only MCP with strict batch, byte, path, schema, and refusal contracts
• deepen authoring, review, testing, debugging, performance, framework, bundle, API-design, reentrancy, flattening, marble, and custom-operator guidance
• retire the standalone migration package, generated local migration skill, and every current repository reference to their package, workspace, and binary names
• reset the active project plan and release train around polyfill, test, agent plugin, and rxjs
• promote the plugin in package, repository, migration, release, and website documentation

Migration assurance

The migration MCP and its engine now have 114 deterministic tests across protocol, engine, behavior, lifecycle, exact candidate output, diagnostics, idempotence, type evidence, and refusal semantics. Packed-artifact testing starts the published stdio bundle, discovers and invokes all four tools, checks text/structured parity, exercises exact limits and malformed inputs, and verifies clean shutdown. Behavioral comparisons are pinned to RxJS 7.8.2.

The release fitness checks also reject any reintroduction of the retired package, workspace path, or executable name.

Validation

• pnpm --filter @rxjs/agent-plugin run test:package
• pnpm --filter @rxjs/observable-polyfill --filter @rxjs/test --filter rxjs run test:package
• pnpm --filter rxjs run test:migration-contracts
• pnpm run test:release:adoption
• pnpm run test:workflows
• pnpm run release:check

The packed plugin contains 124 reviewed files. Release adoption passed for all four packages; the agent-plugin tarball is 1,915,186 bytes under its 2,000,000-byte budget. No model invocation, paid authentication, token-consuming evaluation, or live-model outcome test was run or made a release gate.

Intentional removal

The standalone migration package is deliberately removed in this PR after its deterministic engine, schemas, fixtures, and coverage move into the plugin. Git history remains the recovery path for the retired implementation and archived qualification evidence.